It has become evident that the Clegg Impact Soil Tester is being used in a
variety of ways for compaction control.

Initially its application was restricted to pavement materials such as fine crushed
rocks, gravels and soft rocks but this has been extended to control of earthworks in
general.

The main reasons given for its preference over conventional control by
density testing are usually related to speed of testing, cost of testing and testing
locations, e.g. remote area situations such as mining construction; trench reinstatement;
small job such as parking areas. The main concern and deterrent to its more general use is
associated with interpretation in relation to current specifications based on relative
compaction using the well known Proctor test or derivatives of this.

The commonest procedure is to establish a minimum strength requirement
in terms of Clegg Impact Value (CIV) for the particular moisture conditions, e.g. optimum
moisture content (OMC) for modified Proctor. It is essentially a single value
acceptance/rejection criterion. This target value is generally established for a
particular material on the basis that the resulting decision to accept or reject is the
same as if commonly accepted procedures had been used for the particular situation.

A more sophisticated approach establishes regression equations for CIV
in relation to density and moisture content and uses either judgement or test to determine
the appropriate value for moisture content. This may be by field trials or from laboratory
tests in a CBR mould.

There also appears to be interest in adding an in situ strength
requirement, such as CIV, to the compaction objectives and specification requirements.

Because the Clegg Hammer is becoming widely applied to compaction
control there is a need for discussion on the fundamentals of compaction as a lead up to
answering the commonly asked question - "how does it (the CIV) relate to
Proctor?"

The attached note, BASICS OF COMPACTION CONTROL, outlines some of the
basic concepts relating to compaction control on a strength basis and forms a convenient
starting point.

BASICS OF COMPACTION CONTROL

It should be recognised from the outset that the logical objective of
compaction is not in fact an arbitrarily selected density. The primary objective is the
attainment of a certain minimum strength or compressibility (and sometimes permeability).

Before the Proctor concept was introduced compaction was rather a
haphazard process with no moisture control. In the early 1930s Proctor introduced the
concept of an optimum moisture content (OMC) with its corresponding maximum dry density
(MDD). These conditions could be determined for each soil type in relation to the
compaction effort. The well known standard Proctor test was developed to correspond to
what was known to be suitable for earthworks and later the modified AASHO test was
developed to correspond to the use of heavier vehicles. By compacting at or near the OMC
more efficient use of rollers was achieved. It also became possible to check the
effectiveness of the compaction by relating field density to the MDD achieved in the
laboratory test for the chosen compactive effort.

The Proctor tests recognised two basic levels of strength - one
resulting from 'light' compaction and one from 'heavy' compaction but did not define these
in terms of specific strength parameters. For what may be described as practical reasons
the control of compaction proceeded along the lines of relative compaction, i.e. by the
use of the ratio of field dry density to laboratory maximum dry density. One of the main
practical advantages was that field dry density as a soil property was independent of
moisture content (although moisture content was crucial to the level achieved by the
compaction process). On the other hand strength was dependent for a given soil on both
density (degree of packing) and moisture content (pore water pressures). As a consequence
compaction technology and specifications have evolved largely around relative compaction
and associated field density testing.

The use of direct strength or stiffness measurement for control
introduces the complication that these properties are very much dependent on both moisture
content and density. Further the former not only influences the final strength but also
plays a considerable part in the actual compaction process. However it is not the
gravimetric moisture content value as such that is of concern but rather the form that the
water is in. It may have effect as either capillary water with consequent apparent
cohesion or as free water with the possibility of positive pore air-water pressures. Some
compaction theories consider it is the change from negative to positive pore pressure that
results in the loss of density after optimum moisture content. Also while a dry soil may
exhibit adequate strength this may be reduced to an unacceptable degree by an increase in
moisture content if this is not compensated for by strength resulting from density.

Conversely a wet near saturated soil may be in a critical low stability
state due to positive pore pressure, even though compacted to relatively high density.

During the Proctor and similar impact compaction tests it may be
observed that the sound of the impacts is reflecting the changes in stiffness in the soil
as it is being compacted. By fitting an accelerometer to the hammer as in the case of the
Clegg Hammer the response can be translated into a strength or stiffness parameter via the
peak deceleration. Using this as the strength parameter the onset of strength loss
associated with approaching MDD may be located on the moisture scale and the peak value
may be determined. At this point the pore pressure is about zero so that the impact value
represents mainly the material's strength due to packing of the particles, i.e. due to its
density.

Specifications for compaction control generally take the form of either
method or end result, e.g. by describing the roller size, number of roller passes,
thickness of layer or alternatively by simply requiring a certain minimum percent relative
compaction. The use of method specifications requires also the control of moisture content
- it needs to be at or near the optimum. The use of end result requires moisture control
for efficient achievement of density but the moisture content can be any value for the
actual density determination.

The use of a strength measurement for compaction control has in the
past been by means of various types of penetrometers, bearing tests and falling weight
devices. More recently wheel load deflection measurements by the Benkelman Beam have been
added to proof rolling procedures. Also devices have been fitted to rollers to monitor the
ground stiffness. With all of these methods the difficulty lies in the selection of
appropriate values to be achieved be it in terms of compaction effort, relative
compaction, penetration resistance, deflection, etc. However it is evident that an in
situ strength measurement in some form for compaction control is desirable and is
being sought after.

The Clegg Hammer offers a practical and direct link between laboratory
and field compaction. In its simplest application the selection of the target strength in
terms of CIV can be made for the selected compaction effort applied to the particular soil
at the moisture condition of no pore pressure, positive or negative, i.e. any wetter and
reduction in strength results. It is important that this be for the field compaction
actually used. If the laboratory determined target strength cannot be achieved, the field
effort is inadequate. If much higher, the effort is being wasted - again it must be
emphasised that the moisture condition must be as wet as possible, i.e. for the no
pressure condition otherwise high values may give a false impression of adequate density.
If conditions are such that testing must be performed at moisture contents lower than the
critical value then the actual moisture content needs to be determined by laboratory test
or by judgement. This enables the density to be determined by inference from regression
equations using CIV, density and moisture content. On the other hand if testing above the
critical point the lower CIVs may cause satisfactory work (density wise) to be rejected.

The basic concepts outlined above should be seen as broad
generalisations. The response indicated may be expected to vary in degree from soil to
soil. However it may be seen that the Clegg Impact Test approach is a logical extension of
the Proctor system of compaction control, adding the factor of strength to the design,
construction and testing processes. For typical actual data see Figure 3 and 4.